Method of forming a porous film on a substrate

ABSTRACT

An organic-inorganic hybrid film is deposited on a substrate by introducing, into a vacuum chamber, a gas mixture of a silicon alkoxide and an organic compound and generating a plasma derived from the gas mixture. Then, a hydrogen plasma process is performed with respect to the organic-inorganic hybrid film by introducing, into the vacuum chamber, a gas containing a reducing gas and generating a plasma derived from the gas. As a result, an organic component in the organic-inorganic hybrid film eliminates therefrom and numerous fine holes are formed in hollow portions from which the organic component has eliminated, whereby a porous film composed of the organic-inorganic hybrid film is obtained.

CROSS REFERENCE TO A RELATED PATENT

The subject matter of the present application is a division of Ser. No.09/492,349 now U.S. Pat. No. 6,387,824 filed on Jan. 27, 2000, which isalso assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION

The present invention relates to a method of forming a porous film usedas, e.g., an inter-layer dielectric in a semiconductor integratedcircuit device.

As the integration density of a semiconductor integrated circuit hasincreased, an increased wiring delay time resulting from an increase inwire-to-wire capacitance, which is a parasitic capacitance between metalwires, has presented an obstacle to the implementation of asemiconductor integrated circuit with higher performance. The wiringdelay time is a so-called RC delay which is proportional to the productof the resistance of the metal wire and the wire-to-wire capacitance.

To reduce the wiring delay time, therefore, it is necessary to reducethe resistance of the metal wire or the wire-to-wire capacitance.

As a method of reducing the wire-to-wire capacitance, the reduction ofthe dielectric constant of an inter-layer dielectric formed between themetal wires has been considered. As an inter-layer dielectric having alow dielectric constant, a porous film has been under study as areplacement for a conventional silicon oxide film. It can be said thatthe porous film is only the film capable of providing a dielectricconstant of 2.0 or lower.

In view of the foregoing, there have been proposed various methods offorming porous films.

As a first conventional method of forming a porous film, there has beenknown one wherein a solution of a siloxane polymer precursor containinga thermally unstable organic component is prepared and coated on asubstrate to form a coated film, which is then subjected to a thermalprocess for decomposing and eliminating the organic component such thatnumerous fine holes are formed in hollow portions from which the organiccomponent has eliminated.

As a second conventional method of forming a porous film, there has beenknown one wherein a wet gel is formed on a substrate by coating a silicasol solution on the substrate or by performing CVD and then thecondensation reaction of the silica sol is caused in the wet gel, whilethe volume reduction of the wet gel is suppressed by controlling thespeed at which the solvent eliminates from the wet gel, thus forming theporous film.

As a third conventional method of forming a porous film, there has beenknown a method wherein a solution of silica fine particles is coated ona substrate to form a coated film, which is then sintered such thatnumerous fine holes are formed between the adjacent silica fineparticles.

However, the first conventional method has the problem of higher costsince it is necessary to prepare the solution of the siloxane polymerprecursor. Moreover, since the coated film is formed by coating theprecursor solution on the substrate, the amount of silanol remaining inthe coated film is increased to cause such problems as a degassingphenomenon which is the elimination of moisture or the like in a thermalprocess step performed subsequently and the degradation of the porousfilm resulting from the absorption of moisture by the film.

On the other hand, the second conventional method has the problem ofhigher cost since it requires a special coating apparatus forcontrolling the speed at which the solvent eliminates from the wet gel.Moreover, since a large number of silanol groups remain on the surfacesof the fine holes, they may cause serious degradation of the filmbecause of high moisture absorption, unless they are removed. It istherefore necessary to silylate the surfaces of silanol groups,resulting in a complicated process. In the case of forming the wet gelby CVD, a special CVD apparatus different from a plasam CVD apparatusused normally in a semiconductor process is also required, which alsoincreases cost.

In accordance with the third conventional method, the diameters of thefine holes formed between the adjacent silica fine particles aredetermined by a geometric configuration in which the silica fineparticles are deposited so that the diameters of the fine particles areincreased significantly. Accordingly, it is difficult to adjust thedielectric constant of the porous film to 2 or less.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to solve the forgoingproblems at once and allow the formation of a porous film having adielectric constant of 2 or less in a simple process at low cost.

To attain the above object, a first method of forming a porous filmaccording to the present invention comprises the steps of: depositing anorganic-inorganic hybrid film on a substrate by plasma enhanced CVDusing a gas mixture of a silicon alkoxide and an organic compound as areactive gas; and forming a porous film composed of theorganic-inorganic hybrid film by performing a plasma process using aplasma derived from a gas containing a reducing gas with respect to theorganic-inorganic hybrid film.

In accordance with the first method of forming a porous film, theorganic-inorganic hybrid film is deposited by plasma enhanced CVD usingthe gas mixture of the silicon alkoxide and the organic compound.Accordingly, a low-cost material can be used to deposit theorganic-inorganic hybrid film. Since the plasma process is performed byusing the reducing gas with respect to the organic-inorganic hybrid filmdeposited by plasma enhanced CVD, the decomposed organic componenteliminates and the numerous fine holes are formed in the hollow portionsfrom which the organic compound has eliminated. This ensures theformation of the porous film composed of the organic-inorganic hybridfilm and allows molecular-level control of the diameters of the fineholes in the porous film.

A second method of forming a porous film according to the presentinvention comprises the steps of: depositing an organic-inorganic hybridfilm on a substrate by plasma enhanced CVD using a gas mixture of asilicon alkoxide and an organic compound as a reactive gas; and forminga porous film composed of the organic-inorganic hybrid film byperforming a thermal process with respect to the organic-inorganichybrid film in an atmosphere containing a reducing gas.

In accordance with the second method of forming a porous film, theorganic-inorganic hybrid film is deposited by plasma enhanced CVD usingthe gas mixture of the silicon alkoxide and the organic compound.Accordingly, a low-cost material can be used to deposit theorganic-inorganic hybrid film. Since the thermal process is performedwith respect to the organic-inorganic hybrid film in the atmospherecontaining the reducing gas, the decomposed organic component eliminatesand the numerous fine holes are formed in the hollow portions from whichthe organic component has eliminated. This ensures the formation of theporous film of the organic-inorganic hybrid film and allowsmolecular-level control of the diameters of the fine holes in the porousfilm.

In accordance with the first or second method of forming a porous film,there is formed the porous film composed of the organic-inorganic hybridfilm deposited by plasma enhanced CVD using the gas mixture of thesilicon alkoxide and the organic compound. This obviates the necessityfor a precursor solution, which is indispensable to the deposition of anorganic-inorganic hybrid film by coating, and allows the deposition ofthe organic-inorganic hybrid film using a low-cost material.Consequently, the cost of the porous film is reduced.

Moreover, since the organic component in the organic-inorganic hybridfilm is eliminated by the plasma process using the plasma derived fromthe gas containing a reducing gas or by the thermal process performed inthe gas atmosphere containing the reducing gas and the fine holes areformed in the hollow portions from which the organic component haseliminated, the fine holes having molecular-size diameters can be formedand the dielectric constant of the porous film is reduced reliably.

Furthermore, since the organic-inorganic hybrid film is deposited byplasma enhanced CVD, the amount of remaining silanol is reducedsignificantly compared with an organic-inorganic hybrid film depositedby coating, so that moisture generated from the remaining silanol isreduced significantly. This reduces moisture which will eliminate fromthe porous film in the thermal process subsequently performed as well asvarious troubles resulting from degassing.

In the first or second method of forming a porous film, the siliconalkoxide is preferably an organic silicon alkoxide represented by thegeneral formula: R¹Si(OR²)₃ where R¹ and R² are the same or different,each representing an alkyl group or an aryl group. The arrangementensures the deposition of the organic-inorganic hybrid film by plasmaenhanced CVD using the gas mixture of the organic silicon alkoxide andthe organic compound.

In the first or second method of forming a porous film, the reducing gaspreferably contains a hydrogen gas or an ammonia gas.

In the first or second method, if the reducing gas contains the hydrogengas or the ammonia gas, a silicon atom remaining after the decompositionand elimination of the organic component is terminated by hydrogen, sothat the surfaces of the fine holes formed in the hollow portions fromwhich the organic component has eliminated become hydrophobic. As aresult, moisture is prevented from entering the fine holes so that themoisture absorption resistance of the porous film is increased.

If the reducing gas contains the ammonia gas, in particular, the surfaceof the porous film and the surfaces of the fine holes are nitrided sothat the metal composing the metal film deposited to come in contactwith the porous film is less likely to be diffused in the porous film,which increases the insulation resistance of the porous film.

A third method of forming a porous film according to the presentinvention comprises the steps of: depositing, on a substrate, anorganic-inorganic hybrid film having a siloxane skeleton; and forming aporous film composed of the organic-inorganic hybrid film by performinga plasma process using a plasma derived from a gas containing a reducinggas with respect to the organic-inorganic hybrid film.

In accordance with the third method of forming a porous film, the plasmaprocess using the plasma derived from the gas containing the reducinggas is performed with respect to the organic-inorganic hybrid filmhaving a siloxane skeleton. Consequently, the decomposed organiccomponent eliminates to leave the numerous fine holes formed in thehollow portions from which the organic component has eliminated. Thisensures the formation of the porous film composed of theorganic-inorganic hybrid film and allows molecular-level control of thediameters of the fine holes in the obtained porous film, so that thedielectric constant of the porous film is reduced reliably.

In the third method of forming a porous film, the reducing gaspreferably contains a nitrogen atom.

In the arrangement, the surface of the porous film and the surfaces ofthe fine holes are nitrided so that the metal composing the metal filmdeposited to come in contact with the porous film is less likely to bediffused in the porous film, which increases the insulation resistanceof the porous film. In this case, if the reducing gas contains anammonia gas, the surface of the porous film and the surfaces of the fineholes can be nitrided reliably.

In the third method of forming a porous film, the reducing gaspreferably contains a hydrogen atom.

In the arrangement, a silicon atom remaining after the decomposition andelimination of the organic component is terminated by hydrogen so thatthe surfaces of the fine holes formed in the hollow portions from whichthe organic component has eliminated become hydrophobic. This preventsmoisture from entering the fine holes and increases the moistureabsorption resistance of the porous film.

In this case, if the reducing gas contains a hydrogen gas or an ammoniagas, a silicon atom remaining after the decomposition and elimination ofthe organic component is surely terminated by hydrogen.

A first method of forming a wiring structure according to the presentinvention comprises the steps of: depositing, on a substrate, anorganic-inorganic hybrid film having a siloxane skeleton; forming aresist pattern on the organic-inorganic hybrid film; performing etchingwith respect to the organic-inorganic hybrid film masked with the resistpattern to form a depressed portion composed of a wire groove or acontact hole in the organic-inorganic hybrid film; performing a plasmaprocess using a plasma derived from a gas containing a reducing gas withrespect to the resist pattern and the organic-inorganic hybrid film toremove the resist pattern and form an inter-layer dielectric which is aporous film composed of the organic-inorganic hybrid film; and filling ametal film in the depressed portion of the inter-layer dielectric toform a buried wire or contact composed of the metal film.

In accordance with the first method of forming a wiring structure, theplasma process using the plasma derived from the gas containing thereducing gas is performed with respect to the resist pattern and to theorganic-inorganic hybrid film. This allows the step of removing theresist pattern and the step of forming a porous film composed of theorganic-inorganic hybrid film to be performed simultaneously. As aresult, the porous film composed of the organic-inorganic hybrid filmcan be formed without increasing the number of process steps.

A second method of forming a wiring structure according to the presentinvention comprises the steps of: depositing, on a substrate, a firstorganic-inorganic hybrid film containing an organic component in arelatively low proportion; patterning the first organic-inorganic hybridfilm to form a contact hole in the first organic-inorganic hybrid film;depositing, on the first organic-inorganic hybrid film, a secondorganic-inorganic hybrid film containing an organic component in arelatively high proportion; patterning the second organic-inorganichybrid film to form a wire groove in the second organic-inorganic hybridfilm; filling a metal film in the contact hole and in the wire groove toform a contact and a metal wire each composed of the metal film; andperforming a porous-film forming process with respect to the first andsecond organic-inorganic hybrid films in an atmosphere containing areducing gas to form a first inter-layer dielectric which is a porousfilm composed of the first organic-inorganic hybrid film and a secondinter-layer dielectric which is a porous film composed of the secondorganic-inorganic hybrid film.

In accordance with the second method of forming a wiring structure, theamount of the organic component contained in the secondorganic-inorganic hybrid film is larger than the amount of the organiccomponent contained in the first organic-inorganic hybrid film, so thatthe porosity of the second organic-inorganic hybrid film is higher thanthe porosity of the first organic-inorganic hybrid film. As a result,the second inter-layer dielectric is lower in dielectric constant thanthe first inter-layer dielectric. Moreover, the first inter-layerdielectric is higher in mechanical strength and heat conductivity thanthe second inter-layer dielectric. Accordingly, a wire-to-wire parasiticcapacitance produced between the metal wires formed in the secondinter-layer dielectric is reduced and heat generated in the metal wiresis diffused efficiently into the substrate via the first inter-layerdielectric. Furthermore, since the wiring structure retains sufficientmechanical strength with the first inter-layer dielectric excellent inmechanical strength, it is possible to reduce the dielectric constant ofthe inter-layer dielectric, while retaining the sufficient mechanicalstrength of the wiring structure.

A third method of forming a wiring structure according to the presentinvention comprises: depositing, on a substrate, an organic-inorganichybrid film having a siloxane skeleton; patterning the organic-inorganichybrid film to form a wire groove in the organic-inorganic hybrid film;filling a metal film in the wire groove to form a buried wire composedof the metal film; and performing a plasma process using a plasmaderived from a gas containing a reducing gas with respect to theorganic-inorganic hybrid film to form an inter-layer dielectric which isa porous film composed of the organic-inorganic hybrid film.

In accordance with the third method of forming a wiring structure, theinter-layer dielectric composed of the porous film is formed by formingthe buried wire and then performing the plasma process using thereducing gas with respect to the organic-inorganic hybrid film, so thatthe metal composing the buried wire is prevented from entering the fineholes in the inter-layer dielectric. This prevents an increased leakagecurrent and a short circuit between the buried wires. Since it isunnecessary to form a protecting film in the wiring groove, thewire-to-wire parasitic capacitance between the buried wires can bereduced reliably.

A wiring structure according to the present invention comprises: a firstinter-layer dielectric formed on a substrate and having a contact hole,the first inter-layer dielectric being composed of a porous film havinga relatively low porosity; a second inter-layer dielectric formed on thefirst inter-layer dielectric and having a wire groove, the secondinter-layer dielectric being composed of a porous film having arelatively high porosity; a contact composed of a metal film filled inthe contact hole; and a metal wire composed of a metal film filled inthe wire groove.

In the wiring structure according to the present invention, the porosityof the second inter-layer dielectric is higher than the porosity of thefirst inter-layer dielectric, so that the second inter-layer dielectricis lower in dielectric constant than the second inter-layer dielectricand higher in mechanical strength and heat conductivity than the firstinter-layer dielectric. As a result, it is possible to reduce thedielectric constant of the inter-layer dielectric and retain thesufficient mechanical strength and heat diffusing property of the wiringstructure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and (b) are diagrams of an organic-inorganic hybrid filmobtained by a method of forming a porous film according to a firstembodiment of the present invention;

FIGS. 2(a) and (b) are diagrams of a porous film obtained by the methodof forming a porous film according to the first embodiment;

FIGS. 3(a), (b), and (c) are cross-sectional views illustrating theindividual process steps of a method of forming a wiring structureaccording to a fifth embodiment of the present invention;

FIGS. 4(a), (b), and (c) are cross-sectional views illustrating theindividual process steps of a method of forming a wiring structureaccording to a sixth embodiment of the present invention;

FIG. 5 is a cross-sectional view for illustrating heat diffusion in thewiring structure obtained according to the sixth embodiment;

FIGS. 6(a) and (b) are cross-sectional views for illustrating themechanical strength of the wiring structure obtained according to thesixth embodiment;

FIGS. 7(a), (b), and (c) are cross-sectional views illustrating theindividual process steps of a method of forming a wiring structureaccording to a seventh embodiment of the present invention; and

FIGS. 8(a), (b), and (c) are cross-sectional views illustrating theindividual process steps of the method of forming a wiring structureaccording to the seventh embodiment.

DETAILED DESCRIPTION OF THE INVENTION EMBODIMENT 1

In accordance with a method of forming a porous film according to afirst embodiment of the present invention, an organic-inorganic hybridfilm is formed by plasma enhanced CVD using, as a reactive gas, a gasmixture of a silicon alkoxide gas and an organic compound gas and then aplasma process is performed with respect to the organic-inorganic hybridfilm by using a plasma derived from a gas containing a reducing gas.

First, a gas mixture of a gas obtained by vaporizingvinyltrimethoxysilane (flow rate: 1 ml/min) as the silicon alkoxide andan acetylene gas (flow rate: 100 sccm) as the organic compound gas isintroduced under pressure of 1.0 Torr into a vacuum chamber. On theother hand, RF power of 13.56 MHz is applied with electric power of 500W between counter electrodes to generate a plasma derived from the gasmixture, whereby the organic-inorganic hybrid film having a siloxaneskeleton and a thickness of 320 nm is deposited on a substrate. Theplasma enhanced CVD process was performed at 400° C. for 1 minute.

FIGS. 1(a) and (b) show diagrammatic structures of an organic-inorganicfilm 1, in which 2 denotes the siloxane skeleton and 3 denotes anorganic component.

As a result of measuring the IR absorption spectrum of the depositedorganic-inorganic hybrid film, the organic-inorganic hybrid film wasrecognized as a silicon oxide film containing the organic component. Thedielectric constant of the organic-inorganic hybrid film was measured byCV measurement to be 3.2.

Subsequently, a gas mixture of a hydrogen gas (flow rate: 1000 sccm), anitrogen gas (flow rate: 1000 sccm), and an oxygen gas (flow rate: 100sccm) is introduced under pressure of 4 Torr into the vacuum chamber. Onthe other hand, RF power of 13.56 MHz is applied with electric power of600 W between the counter electrodes to perform a hydrogen plasmaprocess with respect to the organic-inorganic hybrid film. The hydrogenplasma process was performed at 400° C. for 5 minutes.

The IR absorption spectrum of the organic-inorganic hybrid film afterthe hydrogen plasma process was measured, with the result that anabsorption peak derived from the organic component was barely observedand an absorption peak derived from a silicon-hydrogen bond (Si—H) wasincreased. This indicates that the hydrogen plasma process hasdecomposed the organic component and terminated a part of the silicon inthe siloxane skeleton by hydrogen.

According to the first embodiment, the decomposed organic component inthe organic-inorganic hybrid film after the hydrogen plasma processeliminates therefrom and numerous fine holes are formed in hollowportions from which the organic component has eliminated, whereby aporous film is obtained. In this case, since the fine holes are formedin the hollow portions from which the organic component has beenremoved, the diameters of the fine holes can be controlled on themolecular level.

FIGS. 2(a) and (b) show diagrammatic structures of a porous film 5, inwhich 6 denotes the siloxane skeleton and 7 denotes the fine holes. Ascan be seen from the comparison between FIGS. 1(a) and (b) and FIGS.2(a) and (b), the fine holes 7 are formed in the hollow portions fromwhich the organic component 3 has eliminated.

Since the first embodiment has performed the plasma process with respectto the organic-inorganic hybrid film by using the plasma derived fromthe reducing gas, a silicon atom composing an inorganic componentremaining after the decomposition and elimination of the organiccomponent in the organic-inorganic hybrid film is terminated byhydrogen. As a result, surfaces of the fine holes formed in the hollowportions from which the organic component has eliminated becomehydrophobic and moisture is prevented from entering the fine holes.Accordingly, the porous film is less likely to absorb moisture.

Thus, the first embodiment allows molecular-level control of thediameters of the fine holes and low-cost formation of a porous film withhigh moisture absorption resistance by cost-effective plasma enhancedCVD.

In the first embodiment, the step of depositing the organic-inorganichybrid film and the step of forming the porous film composed of theorganic-inorganic hybrid film may be performed by the same plasmaenhanced CVD apparatus or by different plasma enhanced CVD apparatus.

A reduction in the thickness of the porous film obtained in accordancewith the first embodiment was on the order of several percent and thedielectric constant of the porous film was 1.7.

The porosity of the porous film was calculated to be 75% from thedensity thereof. The porosity can be controlled by changing the amountsof vinyltrimethoxysilane and acetylene to be introduced. However, themaximum amount of the acetylene gas that can be introduced is limited tothe range in which the siloxane skeleton can be formed. If the formationof the siloxane skeleton is unsatisfactory due to an excessively largeamount of the organic component introduced, the percentage of filmshrinkage caused by the hydrogen plasma process is increased so that itis difficult to form an excellent porous film.

As the temperature for the hydrogen plasma process becomes higher, thespeed at which the organic component is removed is increased, while thepercentage of film shrinkage tends to increase as well. Although thefirst embodiment has performed the hydrogen plasma process at atemperature of 400° C., a practical removal speed can be achieved evenat a temperature of about 200° C.

EMBODIMENT 2

In accordance with a method of forming a porous film according to asecond embodiment of the present invention, an organic-inorganic hybridfilm is formed by plasma enhanced CVD using, as a reactive gas, a gasmixture of a silicon alkoxide gas and an organic compound gas and then athermal process is performed with respect to the organic-inorganichybrid film in an atmosphere containing a reducing gas.

First, the organic-inorganic hybrid film is formed by plasma enhancedCVD using, as the reactive gas, a gas mixture of a gas obtained byvaporizing vinyltrimethoxysilane as the silicon alkoxide and anacetylene gas as the organic compound gas, similarly to the firstembodiment.

Next, a thermal process at a temperature of about 400° C. is performedwith respect to the organic-inorganic hybrid film by using, e.g., anelectric furnace in an atmosphere containing a reducing gas which iscomposed of a gas mixture of a hydrogen gas, a nitrogen gas, and anoxygen gas.

According to the second embodiment, the organic component is decomposedby the thermal process in the atmosphere containing the reducing gas toeliminate and numerous fine holes are formed in hollow portions fromwhich the decomposed organic component has eliminated, whereby a porousfilm is obtained. In this case, since the fine holes are formed in thehollow portions from the organic component has been removed, thediameters of the fine holes can be controlled on the molecular level.

Moreover, since the second embodiment has performed the thermal processwith respect to the organic-inorganic hybrid film in the atmospherecontaining the reducing gas, a silicon atom composing an inorganiccomponent remaining after the decomposition and elimination of theorganic component is terminated by hydrogen. Accordingly, the porousfilm is less likely to absorb moisture.

Thus, the second embodiment allows molecular-level control of thediameters of the fine holes and low-cost formation of a porous film withhigh moisture absorption resistance by cost-effective plasma enhancedCVD.

EMBODIMENT 3

In accordance with a method of forming a porous film according to athird embodiment of the present invention, an organic-inorganic hybridfilm is formed by plasma enhanced CVD using, as a reactive gas, a gasmixture of a silicon alkoxide gas and an organic compound gas and then aplasma process is performed with respect to the organic-inorganic hybridfilm using a plasma derived from an ammonia gas and an oxygen gas.

First, the organic-inorganic hybrid film is formed by plasma enhancedCVD using, as the reactive gas, a gas mixture of a gas obtained byvaporizing vinyltrimethoxysilane as the silicon alkoxide and anacetylene gas as the organic compound gas, similarly to the firstembodiment.

Next, a gas mixture of an ammonia gas (flow rate: 100 sccm), an argongas (flow rate: 1000 sccm) as a dilute gas, and an oxygen gas (flowrate: 100 sccm) is introduced under pressure of 5 Torr into a vacuumchamber and RF power of 13.56 MHz is applied with electric power of 500W between counter electrodes to perform an ammonia plasma process withrespect to the organic-inorganic hybrid film. The ammonia plasma processis performed at a temperature of 400° C. for 10 minutes.

According to the third embodiment, the organic component is decomposedby the ammonia plasma process to eliminate and numerous fine holes areformed in hollow portions from which the decomposed organic componenthas eliminated, whereby a porous film is obtained.

Moreover, since the third embodiment has performed the ammonia plasmaprocess, a silicon atom composing an inorganic component remaining afterthe decomposition and elimination of the organic component is nitrided.Accordingly, the porous film is less likely to absorb moisture.

Since the third embodiment also allows the formation of theorganic-inorganic hybrid film by cost-effective plasma enhanced CVD,cost can be reduced.

The dielectric constant of the porous film obtained in accordance withthe third embodiment was 1.9, which is slightly higher than thedielectric constant of the porous film obtained in accordance with thefirst embodiment.

To examine the diffusion of copper in the porous film obtained inaccordance with the third embodiment, a thin film of copper wasdeposited on the porous film and subjected to a thermal process at atemperature of 400° C. Thereafter, the degradation of the insulationresistance of the porous film was examined. However, the degradation ofthe insulation resistance was not observed and it was proved that copperwas barely diffused.

Although each of the first to third embodiments has usedvinyltrimethoxysilane as the silicon alkoxide, an organic siliconalkoxide represented by the general formula: R¹Si(OR²)₃ where R¹ and R²are the same or different, each representing an alkyl group or an arylgroup may also be used generally instead of vinyltrimethoxysilane.

As an example of the organic silicon alkoxide represented by the generalformula: R¹Si(OR²)₃, there may be listed methyltrimethoxysilane(R₁:methyl group, R₂:methyl group), phenyltrimethoxysilane (R₁:phenylgroup, R₂:methyl group), or ethyltrimethoxysilane (R₁:ethyl group,R₂:methyl group). In such an organic silicon alkoxide, R₂ may alsorepresent an ethyl group or a phenyl group, instead of a methyl group.

Although each of the first to third embodiments has used acetylene asthe organic compound gas, it is also possible to use varioushydrocarbons such as hydrocarbon fluoride or compounds of carbonfluoride. Specifically, there can be listed a saturated or unsaturatedcompound of perfluorocarbon, hydrofluorocarbon, or hydrocarbon.

Although each of the first to third embodiments has used, as thereactive gas, the gas mixture of the gas obtained by vaporizing thesilicon alkoxide and the organic compound gas, it is also possible touse a single gas obtained by vaporizing an alkoxysilane having anorganic silicon bond, such as vinyltrimethoxysilane. The use of such asingle film also allows the formation of the porous film, though theporosity of the film is reduced.

EMBODIMENT 4

In accordance with a method of forming a porous film according to afourth embodiment, a plasma process is performed with respect to anorganic-inorganic hybrid film having a siloxane skeleton by using aplasma derived from a gas containing a reducing gas.

First, a coated film obtained by coating, on a substrate, phenylsiloxane as a solution having a siloxane skeleton and an organiccomponent is subjected to a thermal process at a temperature of 400° C.,whereby an organic SOG film is obtained.

As a result of measuring the IR absorption spectrum of the organic SOGfilm, an absorption peak derived from a siicon-phenyl bond was observed.The dielectric constant of the organic SOG film was measured to be 3.1by CV measurement.

Next, a gas mixture of a hydrogen gas (flow rate: 1000 sccm), a nitrogengas (flow rate: 1000 sccm), and an oxygen gas (flow rate: 100 sccm) wasintroduced under pressure of 4 Torr into a vacuum chamber, while RFpower of 13.56 MHz was applied with electric power of 500 W betweencounter electrodes, whereby a hydrogen plasma process was performed withrespect to the organic-inorganic hybrid film. The hydrogen plasmaprocess was performed at a temperature of 400° C. for 5 minutes.

The IR absorption spectrum of the organic SOG film after the hydrogenplasma process was measured, with the result that the absorption peakderived from a silicon-phenyl bond was not observed at all, while anabsorption peak derived from a silicon-hydrogen bond was observed. Thisindicates that the hydrogen plasma process has decomposed the organiccomponent and terminated a part of the silicon in the siloxane skeletonby hydrogen.

The dielectric constant of the porous film obtained in the fourthembodiment was 1.9.

Thus, the fourth embodiment allows molecular-level control of thediameters of fine holes and low-cost formation of a porous film withhigh moisture absorption resistance.

Although the fourth embodiment has used an organic SOG film composed ofphenylsiloxane as the organic-inorganic hybrid film having a siloxaneskeleton, it is not limited to the organic SOG film.

A porous film may also be formed by performing a thermal process at atemperature of 400° C. with respect to the organic SOG film obtained inaccordance with the same method as used in the fourth embodiment in anatmosphere containing a reducing gas which is composed of a hydrogengas, a nitrogen gas, and an oxygen gas, similarly to the secondembodiment.

Alternatively, a porous film may be formed by performing an ammoniaplasma process with respect to the organic SOG film obtained inaccordance with the same method as used in the fourth embodiment,similarly to the third embodiment.

EMBODIMENT 5

A method of forming a wiring structure according to a fifth embodimentof the present invention will be described with reference to FIGS. 3(a),(b), and (c).

First, as shown in FIG. 3(a), an organic-inorganic hybrid film 11 havinga siloxane skeleton and a thickness of, e.g., 600 nm is deposited on asemiconductor substrate 10 by the same plasma enhanced CVD as performedin the first embodiment. Then, a resist pattern 12 having an openingover the region of the organic-inorganic hybrid film 11 in which a wiregroove or contact hole is to be formed is formed on theorganic-inorganic hybrid film 11.

Next, as shown in FIG. 3(b), dry etching is performed with respect tothe organic-inorganic hybrid film 11 masked with the resist pattern 12,thereby forming a patterned organic-inorganic hybrid film 11A having adepressed portion 13 composed of a wire groove or contact hole.

Next, the same hydrogen plasma process as performed in the firstembodiment is performed with respect to the patterned organic-inorganichybrid film 11A and the resist pattern 12 to remove the resist pattern12 and form an inter-layer dielectric 14 which is a porous film composedof the patterned organic-inorganic hybrid film 11A.

Next, a metal film 15 such as a copper film is filled in the depressedportion of the inter-layer dielectric 14 such that a buried wire orcontact composed of the metal film 15 is formed.

According to the fifth embodiment, the hydrogen plasma process allowsthe removal of the resist pattern 12 and the formation of the porousfilm composed of the patterned organic-inorganic hybrid film 11A to beperformed simultaneously, so that the number of process steps forforming the wiring structure is reduced.

EMBODIMENT 6

A method of forming a wiring structure according to a sixth embodimentof the present invention will be described with reference to FIGS. 4(a),(b), and (c).

First, as shown in FIG. 4(a), a first organic-inorganic hybrid film 21having a siloxane skeleton and a thickness of, e.g., 600 nm is depositedon a semiconductor substrate 20 by the same plasma enhanced CVD asperformed in the first embodiment. Then, a second organic-inorganichybrid film 22 having a siloxane skeleton and a thickness of, e.g., 300nm is deposited on the first organic-inorganic hybrid film 21 by thesame plasma enhanced CVD as performed in the first embodiment.

The sixth embodiment is characterized in that the amount of the organiccomponent contained in the second organic-inorganic hybrid film 22 isadjusted to be larger than the amount of the organic component containedin the first organic-inorganic hybrid film 21. Specifically, the molarmixture ratio of an organic compound gas (acetylene gas) to a siliconalkoxide (vinyltrimethoxysilane) gas in a gas mixture introduced into avacuum chamber is set to, e.g., 0.2 during the deposition of the firstorganic-inorganic hybrid film 21, while it is set to 0.5 during thedeposition of the second organic-inorganic hybrid film 22. If the molaramount of the organic compound gas is A and the molar amount of thesilicon alkoxide gas is B, the molar mixture ratio is defined asA/(A+B). Accordingly, the amount of the organic component to becontained in the first organic-inorganic hybrid film is reduced duringthe deposition thereof, while the amount of the organic component to becontained in the second organic-inorganic hybrid film is increasedduring the deposition thereof.

Next, as shown in FIG. 4(b), the first and second organic-inorganichybrid films 21 and 22 are patterned such that a contact hole 23 isformed in the first organic-inorganic hybrid film 21 and a wire groove24 is formed in the second organic-inorganic hybrid film 21.

The method of forming the contact hole 23 in the first organic-inorganichybrid film 21 and forming the wire groove 24 in the organic-inorganichybrid film 22 is not particularly limited. It is possible to form thecontact hole 23 in the first organic-inorganic hybrid film 21,depositing the second organic-inorganic hybrid film 22, and then formingthe wire groove 24 in the second organic-inorganic hybrid film 22.Alternatively, it is also possible to sequentially deposit the first andsecond organic-inorganic hybrid films 21 and 22 and then forming thecontact hole 23 in the first organic-inorganic hybrid film 21, whileforming the wire groove 24 in the second organic-inorganic hybrid film22.

Next, the same hydrogen plasma process as performed in the firstembodiment is performed with respect to the first and secondorganic-inorganic hybrid films 21 and 22, whereby porous films composedof the first and second organic-inorganic hybrid films 21 and 22 areformed to serve as first and second inter-layer dielectrics 25 and 26.As stated previously, since the amount of the organic component in thesecond organic-inorganic hybrid film 22 is larger than the amount of theorganic component in the first organic-inorganic hybrid film 21, theporosity of the second inter-layer dielectric 26 is higher than that ofthe first inter-layer dielectric 25. Accordingly, the dielectricconstant of the second inter-layer dielectric 26 is lower than that ofthe first inter-layer dielectric 25.

Thereafter, a copper film, e.g., is filled in the contact hole 23 in thefirst inter-layer dielectric 25 and in the wire groove 24 in the secondinter-layer dielectric 26 to form the wiring structure in a dualdamascene configuration consisting of a contact 27 and a metal wire 28.

According to the sixth embodiment, the porosity of the first inter-layerdielectric 25 is 50% and the dielectric constant thereof is 2.3. On theother hand, the porosity of the second inter-layer dielectric 26 is 85%and the dielectric constant thereof is 1.7.

Since the dielectric constant of the second inter-layer dielectric 26filled in the metal wires 28 is 1.7 and extremely low in the sixthembodiment, a wire-to-wire parasitic capacitance produced between metalwires 28 is reduced. Although the dielectric constant of the firstinter-layer dielectric 25 is 2.3 and slightly higher, the problem of anincreased wire-to-wire parasitic capacitance produced between the metalwires 28 does not occur since the metal wires 28 are not formed in thefirst inter-layer dielectric 25.

FIG. 5 is a cross-sectional view for illustrating the excellent heatconductivity of a multilayer wiring structure obtained in accordancewith the second embodiment, in which the first and second inter-layerdielectrics 25 and 26 are alternately formed on the semiconductorsubstrate 20. The contacts 27 are formed in the individual firstinter-layer dielectrics 25, while the metal wires 28 are formed in theindividual second inter-layer dielectrics 26.

Since the first inter-layer dielectric 25 is low in air content, theheat conductivity thereof is close to that of a silicon oxide film whichhas been used conventionally. Accordingly, a heat conduction path asindicated by the arrow in FIG. 5 is provided so that heat generatedduring the operation of an integrated circuit device is transmitted tothe semiconductor substrate 20 and diffused efficiently. That is, if theinter-layer dielectrics are composed only of porous films having a highporosity, heat is vertically conducted via the metal wires 28 in theupper-layer portions of the inter-layer dielectrics in which the metalwires 28 are formed, while heat is less likely to be conducted in thelower portions of the inter-layer dielectrics in which the contacts 27are formed, since the metal wires 28 are not present. In the multilayerwiring structure obtained in accordance with the sixth embodiment, onthe other hand, the lower portions of the inter-layer dielectrics inwhich the contacts 27 are formed are composed of the first inter-layerdielectrics 25 excellent in heat conductivity, so that vertical heatconduction is not interrupted in the lower portions of the inter-layerdielectrics.

FIGS. 6(a) and (b) are cross-sectional views for illustrating themechanical strength of the multilayer wiring structure obtained inaccordance with the sixth embodiment, which is not inferior to themechanical strength of a conventional multilayer wiring structure. Asshown in the drawings, the first and second inter-layer dielectrics 25and 26 are alternately formed on the semiconductor substrate 20. Thecontacts 27 are formed in the individual first inter-layer dielectrics25, while the metal wires 28 are formed in the individual secondinter-layer dielectrics 26.

Since the air content of each of the first inter-layer dielectrics 25 is50%, the mechanical strength thereof is approximately equal to that ofthe silicon oxide film which has been used conventionally. By contrast,the air content of each of the second inter-layer dielectrics 26 is 85%so that the mechanical strength thereof is inferior to that of thesilicon oxide film.

Nevertheless, the multilayer wiring structure obtained in accordancewith the sixth embodiment is not inferior in mechanical strength to theconventional multilayer wiring structure using the silicon oxide filmsas the inter-layer dielectrics. Specifically, the hatched portions inFIG. 6(b) represent regions which are approximately equal in mechanicalstrength to the conventional multilayer wiring structure, while thedotted portions in FIG. 6(b) represent regions which are inferior inmechanical strength to the conventional wiring structure. As can be seenfrom FIG. 6(b), the regions which are inferior in mechanical strengthare only those portions of the second inter-layer dielectrics 26interposed between the metal wires 28, which are discontinued in eitherof horizontal and vertical directions. In other words, the regions whichare excellent in mechanical strength are continued in either ofhorizontal and vertical directions, so that the multilayer wiringstructure obtained in accordance with the sixth embodiment is notinferior in mechanical strength to the conventional multilayer wiringstructure.

EMBODIMENT 7

A method of forming a wiring structure according to a seventh embodimentof the present invention will be described with reference to FIGS. 7(a),(b), and (c) and 8(a), (b), and (c)

First, as shown in FIG. 7(a), an organic-inorganic hybrid film 31 havinga siloxane skeleton and a thickness of, e.g., 400 nm is deposited on asemiconductor substrate 30 by the same plasma enhanced CVD as performedin the first embodiment. Then, atitanium nitride film 32 having athickness of, e.g., 50 nm is deposited on the organic-inorganic hybridfilm 31 by sputtering. Subsequently, a silicon oxide film 33 having athickness of, e.g., 100 nm is deposited on the titanium nitride film 32by sputtering and a resist pattern 34 having an opening over the regionof the silicon oxide film 33 in which a wire is to be formed is formedon the silicon oxide film 33.

Next, as shown in FIG. 7(b), dry etching is performed with respect tothe silicon oxide film 33 masked with the resist pattern 34 to form apatterned silicon oxide film 33A having the opening over the wireformation region. Then, as shown in FIG. 7(c), the resist pattern 34 isremoved by ashing. Thereafter, dry etching is performed with respect tothe titanium nitride film 32 masked with the patterned silicon oxidefilm 33A to form a hard mask 32A composed of the titanium nitride film32.

Next, as shown in FIG. 8(a), dry etching is performed with respect tothe organic-inorganic hybrid film 31 by using the hard mask 32A to forma patterned organic-inorganic hybrid film 31A having a wire groove 35.It is to be noted that the patterned silicon oxide film 32A is removedin the dry etching step performed with respect to the organic-inorganichybrid film 31.

Next, as shown in FIG. 8(b), a metal film such as a copper film isdeposited on the patterned organic-inorganic hybrid film 31A to fill inthe wire groove 35. After that, the metal film and the hard mask 32Aoverlying the patterned organic-inorganic hybrid film 31A are removed byCMP so that a buried wire 36 composed of the copper film is formed.

Next, the same hydrogen plasma process as performed in the firstembodiment is performed with respect to the patterned organic-inorganichybrid film 31A, whereby an inter-layer dielectric 37 composed of thepatterned organic-inorganic hybrid film 31A is formed, as shown in FIG.8(c).

A description will be given below to the characteristics of the methodof forming a wiring structure according to the seventh embodiment.

First, since the inter-layer dielectric 37 composed of a porous film isformed by forming the buried wire 36 and then performing the hydrogenplasma process with respect to the patterned organic-inorganic hybridfilm 31A, the metal composing the buried wire 36 is prevented fromentering the fine holes of the porous film, which prevents an increasedleakage current and a short circuit between the buried wires 36.Moreover, since it is unnecessary to form a protecting film composed ofa silicon oxide film or the like on the wall faces of the wire grooveand thereby prevent the metal film filled in the wire groove 35 fromentering the portion of the porous film exposed in the wire groove, thewire-to-wire capacitance can surely be reduced.

Second, since CMP is performed with respect to the metal wire before theformation of the porous film composed of the patterned organic-inorganichybrid film 31A, the surface of the metal film filled in the wire groove35 which is in contact with the patterned organic-inorganic hybrid film31A becomes flat, which improves the orientation property of the metalcomposing the metal film and therefore there liability of the buriedwires 36. Moreover, since CMP is performed with respect to theorganic-inorganic hybrid film having a higher mechanical strength than aporous film, there can be prevented a situation where an inter-layerdielectric composed of a porous film is destroyed in the CMP step.Furthermore, since a polishing slurry and fine particles contained inthe slurry are prevented from entering the fine holes of the porous filmin the CMP step, there can be prevented the degradation of theinter-layer dielectric and the generation of particles.

What is claimed is:
 1. A method of forming a porous film, comprising thesteps of: (a) depositing, on a substrate, an organic-inorganic hybridfilm having a siloxane skeleton; and (b) forming a porous film composedof said organic-inorganic hybrid film.
 2. The method of claim 1, whereinsaid step (a) includes depositing said organic-inorganic hybrid film onsaid substrate by plasma enhanced CVD using a gas mixture of a siliconalkoxide and an organic compound as a reactive gas; and said step (b)includes forming said porous film composed of said organic-inorganichybrid film by performing a plasma process using a plasma derived from agas containing a reducing gas with respect to said organic-inorganichybrid film.
 3. The method of claim 2, wherein said silicon alkoxide isan organic silicon alkoxide represented by the general formula:R¹Si(OR²)₃ where R¹ and R₂ are the same or different, each representingan alkyl group or an aryl group.
 4. The method of claim 2, wherein saidreducing gas contains a hydrogen gas or an ammonia gas.
 5. The methodclaim 1, wherein step (a) includes depositing said organic-inorganichybrid film on said substrate by plasma enhanced CVD using a gas mixtureof a silicon alkoxide and an organic compound as a reactive gas; andsaid step (b) includes forming said porous film composed of saidorganic-inorganic hybrid film by performing a thermal process withrespect to said organic-inorganic hybrid film in an atmospherecontaining a reducing gas.
 6. The method of claim 5, wherein saidsilicon alkoxide is an organic silicon alkoxide represented by thegeneral formula: R¹Si(OR²)₃ where R¹ and R² are the same or different,each representing an alkyl group or an aryl group.
 7. The method ofclaim 5, wherein said reducing gas contains a hydrogen gas or an ammoniagas.
 8. The method of claim 1, wherein said step (a) includes deposingsaid organic-inorganic hybrid film by applying a solution having saidsiloxane skeleton and including organic components on said substrate;and said step (b) includes forming said porous film composed of saidorganic-inorganic hybrid film by performing a plasma process using aplasma derived from a gas containing a reducing gas with respect to saidorganic-inorganic hybrid film.
 9. The method of claim 8, wherein saidreducing gas contains a nitrogen atom.
 10. The method of claim 8,wherein said reducing gas contains an ammonia gas.
 11. The method ofclaim 8, wherein said reducing gas contains a hydrogen atom.
 12. Themethod of claim 8, wherein said reducing gas contains a hydrogen gas oran ammonia gas.
 13. The method of claim 8, wherein said solution havingsaid siloxane skeleton and including organic components is composed ofphenylsiloxane.